The use of attenuated Salmonella typhi strain Ty21a as an effective live oral vaccine against typhoid fever in humans is well documented. The possibility that a live oral typhoid vaccine also will serve as a carrier of foreign antigens from a wide variety of pathogens is based on reports that Ty21a stimulates a broad range of humoral and cellular immune responses. Because of many practical and theoretical drawbacks, strain Ty21a is considered far from optimal for use as an antigen carrier. Attenuated strains of S. typhi, for use as a human typhoid vaccine and antigen carrier, should possess two well-separated and well-defined deletion mutations and should be highly immunogenic after a single dose. Strains of Salmonella harboring a single aro mutation are remarkably attenuated but highly immunogenic in animals. S. typhi strains that carry both aro and pur mutations were shown to be safe but non-immunogenic in volunteers and hence hyperattenuated. The pur mutation, included as a safety factor, was later shown to cause the hyperattenuation. Because the aro gene on the Salmonella chromosome are widely separated, mutations in two of the aro genes can be exploited to introduce safety against the chance of in vivo reversion as was attempted with the aro pur mutants of S. typhi. It is proposed herein to use aro mutations as the sole attenuating marker of S. typhi with the aim of constructing a live oral typhoid vaccine that is more suitable as an antigen carrier. To this end, in vitro DNA techniques were used to construct defined deletions in the aroC and aroD genes of S. typhi strain Ty2. The safety and immunogenicity of strains carrying defined double deletion mutations will be assessed in volunteers. Then by means of in vitro DNA techniques, the double aro mutants of S. typhi will be used to construct stable hybrid strains expressing, from the chromosome of the carrier, carefully selected cloned antigens of enterotoxigenic Escherichia coli, Salmonella typhimurium, Rickettsia prowazekii, or Plasmodium falciparum. These constructs will be used to investigate, in volunteers, the capacity of the carrier to elicit immune responses specific for the cloned antigen such as: secretory IgA, serum antibody, IgA-armed CD+-mediated bacterial killing, and CD+ lymphocytes manifesting proliferative, lymphokine-producing or cytotoxic phenotypes. To conclude, it is the aim of this research proposal to construct a live oral typhoid vaccine that will be more suitable as an antigen carrier. Hybrid constructs then will be used to provide information on the capacity of the carrier to elicit a broad range of immune responses against cloned antigens.